In an electric arc welder, an electro mechanical contactor or switch is used to direct welding power from the power source to the contact tip of the welding gun. The electro mechanical contactor relies upon mechanical contacts to turn the welding current on and off; consequently, arcing at the contacts leads to degeneration of contactor performance. The degeneration of performance is exhibited as an increased contact resistance that causes an equal decrease in output power. With continued use of degraded mechanical contacts, the contactor will ultimately overheat. Such deteriorating contactors must be replaced prior to ultimate failure. If the contactor fails during an actual welding operation, the weld performed during the welding cycle may require extensive rewelding or other corrective measures.
Since the commonly used electro mechanical contactor includes a single contactor operated by a solenoid responsive to the position of the weld trigger of the welding gun, a suggested, but not prior art, solution to the overheating of the mechanical contacts is to provide two current branches in the wire feeder. Two sets of contacts are each operated by a solenoid. This suggested solution to the problem of contactor overheating was ineffective. One of the contacts would become overheated thereby driving more current into the other contactor causing it to overheat rapidly. This unbalance of current was caused by the dynamics of the two solenoids operated in unison by the signal from the welding trigger. The concept of providing two separate branches with individual sets of mechanical contacts operated by a dedicated solenoid proved to be no better than the existing mechanical contact or device. The attempt to solve the deficiency merely increased the cost of the contactor device. For that reason, this suggestion was not implemented in the welding industry. The industry turned to the concept disclosed in publication US 2004/0089645, incorporated by reference herein. A solid state circuit was used to prevent overheating and deterioration of the contactor in the wire feeder. When a solid state switch is employed, there is no contact created arcing. Furthermore, there is no mechanical contacts to wear. But, the conduction resistance of the solid state switch is substantially higher than the normal contact resistance of closed mechanical contacts. Indeed, the resistance of a solid state switch is often in the general range of 0.50 ohms or greater. Since an electric arc welder often operates at current of over 300 amperes, a substantial amount of heat energy is generated at the solid state switch or switches. This generated heat must be dissipated in the wire feeder cabinet. Internal heat dissipation is challenging for wire feeders requiring generally sealed cabinets, especially when used in the field.
Thus, the solid state switch approach is costly and not completely satisfactory.